In a torrefaction process, biomass is heated during which biomass properties are changed to obtain a much better fuel quality for combustion and gasification applications. Biomass is present in biodegradable industrial and domestic waste such as e.g. agricultural waste, wood chippings, mowed grass and even municipal solid waste, and is widely viewed of as a good alternative for fossil fuels.
Unfortunately the current energy infrastructure is based on coal fired plants, and biomass currently cannot be fired or even co-fired in such plants, because the properties of biomass differ significantly from those of coal. For example, biomass tends to be difficult to pulverize and its energy density is typically substantially lower than the energy density of coal. The latter also makes logistics and storage of biomass relatively expensive.
In order to co-fire biomass in existing coal-fired plants, the biomass can be treated to alter its properties to be more coal-like. Torrefaction is a thermal pre-treatment method for biomass which can be applied to all kinds of biomass. During the torrefaction process, the biomass is usually heated under atmospheric pressure to temperatures of about 200-320° C. in the substantial absence of oxygen. The oxygen depletion prevents combustion of the biomass, while at the same time the high temperature leads to the removal of water and volatile organic compounds from the biomass. After the process, the biomass has decreased in weight by up to 30%, whilst the energy value has only been reduced by e.g. 10%. Thus, a product with a higher calorific value is achieved.
Torrefaction drives moisture, and oxygen- and hydrogen rich functional groups from the crude biomass. As a result, the properties of torrefied biomass are very similar irrespective of the source of the crude biomass. Furthermore, torrefaction eliminates any biological activity, so that biological decomposition like rotting does not occur. Torrefaction also eliminates the risk of spontaneous combustion. The resulting torrefied materials are more brittle, leading to better grindability, and the torrefied biomass is also hydrophobic, which makes storage in open air feasible. Several modes exist for torrefying biomass. For example, the biomass can be heated in a compact moving bed reactor, a belt dryer, a rotating drum, or a fluidized bed reactor.
After torrefaction, the biomass should be cooled to decrease the temperature from reactor conditions (200 to 320° C.) to below 100° C. This stops the decomposition reactions that torrefaction represents and conditions the torrefied product for further upgrading such as milling and densification. A potential hazard that can occur when cooled torrefied biomass is exposed to air is spontaneous ignition. Especially when the particles are still hot (>50° C.) and bone dry.
Direct cooling of the product with water is a very effective manner to reduce the temperature quickly and add a significant amount of moisture. This method yields a high cooling rate of the product particles, which is attractive to freeze the decomposition reactions and reduce volatile emissions from the particles in the cooler or even further downstream. It also eliminates fire hazards.
Such a cooling process comprising a step of applying water onto the hot torrefied biomass is known from WO2012158112-A2, for example. Water is applied to the torrefied material to quench-cool the material, after which both the torrefied material and the gases that are released during cooling are fed to a common cooling device for further cooling the torrefied material together with these gasses. In this way components of these gases condense in the cooling device, e.g. on the torrefied material.
The same principle is applied in the cooling step of US20140208995-A1. In an embodiment of US20140208995-A1 biomass exits a torrefaction reactor via a biomass outlet and is thereafter quench cooled in a water application device. The torrefied material that has passed the water application device is fed to a screw cooler for further cooling the torrefied material together with torrefaction gases such that part of the components of the torrefaction gases condense in the screw cooler. Non-condensed components are sucked out of the cooling screw by means of a fan and transferred back to the biomass outlet. Only the initial application of water provides for fast cooling. The temperature during this initial cooling step is however not reduced below 120° C. The further cooling process in the screw cooler is much slower.
WO2012158111-A1 describes a cooling process for torrefied biomass, in which hot torrefied biomass is transported through a cooled drum by means of a screw. At least a part of the cooled torrefied material is mixed-back with hot torrefied material during cooling. In this way, gasses given off by the hot material will condense on the cold torrefied material. This decreases clogging of the device, increases the energy yield of the final torrefied product, and increases the hydrophobicity of the final torrefied product. Only the initial application of water provides for fast cooling. The cooling process in the common cooling device is much slower.
WO2013081510-A1 describes cooling of torrefied material by adding water. A cyclonic separator to separate the steam and torrefied material is disclosed. After this separation step, the steam is condensed for recovery of heat.
EP2589648-A1 discloses a process for the torrefaction of biomass, in which processed biomass is cooled by spraying water onto the product. EP2589648-A1 specifically mentions not raising the moisture content of the cooled product above approximately 3 weight percent.
WO2011119470-A1 also discloses cooling of hot torrefied biomass by a water spray. It furthermore discloses conditioning the biomass to a moisture content of 5 to approximately 15% for lubrication purposes during densification.
In US 2012/0073159 A1, cellulosic material is cascaded from top to bottom between a plurality of rotatable trays vertically stacked within multiple processing zones of an apparatus. The hot torrefied cellulosic material is wiped off the bottom tray into a volume of water. In an embodiment exhaust gases are condensed, and water and other condensables are removed through an outlet. The remaining exhaust gases are provided to a burner and heat exchanger before they are vented.